Tomato Flavor

When we bite into a tomato, what our brains register as a “tomato” is actually a complex interaction between sugars (glucose and fructose), acids (citric, malic and ascorbic) and multiple volatile compounds. While several hundred volatiles have been identified in tomato, only about 15-20 actually impact our perception of the fruit. This is because most of these compounds fall below the odor threshold. This threshold is determined by both the concentration of the substance and our ability to detect it. Thus, a compound that is present in quite high levels that we detect poorly will not register. Conversely, a substance to which we are quite sensitive will be perceived in very low amounts. For an excellent and understandable explanation of this concept, see Baldwin et al. HortScience 35:1013-1022 (2000). Much work has gone into defining the “important” volatiles that together make up tomato flavor and the list provided below should be considered only an approximation. Odor thresholds vary markedly between individuals and can be greatly influenced by the way in which the volatile is presented. A list of the major volatiles, their concentrations and their characteristic odors follows:

Volatile 
Conc. (ppb)
Log odor units
Odor Characteristics
cis-3-Hexenal 
12,000 
3.7
tomato/green
ß-ionone
2.8
fruity/floral
hexanal   
3,100 
2.8
green/grassy
ß-damascenone 
2.7
fruity
1-penten-3-one
520 
2.7
fruity floral/green
2+3-methylbutanal 
27 
2.1
musty
trans-2-hexenal  
270 
1.2
green
2-isobutylthiazole  
36 
1.0
tomato vine
1-nitro-2-phenylethane 
17 
0.9
musty, earthy
trans-2-heptenal 
60 
0.7
green
phenylacetaldehyde  
15 
0.6
floral/alcohol
6-methyl-5-hepten-2-one 
130 
0.4
fruity, floral
cis-3-hexenol   
150 
0.3
green
2-phenylethanol  
1,900 
0.3
nutty/fruity
3-methylbutanol       
380 
0.2
earthy, musty
methyl salicylate
48 
0.08 
wintergreen

   

Tomato CCD genes

Several of these volatiles (ß-ionone, ß-damascenone and 6-methyl-5-heptene-2-one) are produced by oxidative cleavage of carotenoids. Additional carotenoid-derived volatiles just below this threshold include geranyl acetone and pseudoionone. We have isolated a pair of closely related tomato carotenoid cleavage dioxygenases (CCDs) that cleave a wide range of carotenoid substrates at the 9,10 and 9’,10’ positions to release a variety of volatile compounds that are essential components of tomato flavor. These results provide a direct link between enzyme and volatile.

Carotenoid Cleavage Compounds

The 2-Phenylethanol pathway

Several important flavor volatiles (methylsalicylate, phenylacetaldehyde, 2-phenylethanol and 1-nitro-2-phenethane) are derived from the shikimate pathway. We have isolated several of the genes responsible for synthesis of these important volatiles. The most important enzymes for the phenylalanine-derived volatiles are the aromatic amino acid decarboxylases (AADCs).  They perform the first step in the pathway: conversion of phenylalanine to phenethylamine. For methylsalicylate, a methyltransferase, SAMT, is the most important enzyme.

One complicating factor that is not well understood involves conjugation of the flavor volatiles to sugars such as glucose. These glycosides are no longer volatile and therefore cannot contribute directly to flavor.

aromatic volatiles pathways

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